1. Field of the Invention
The present invention relates to a feeding structure of an antenna device formed on a window glass panel of a motor vehicle and an antenna device for a motor vehicle.
2. Related Art
Where an antenna for a band width of 1 GHz or more is formed on a window glass panel of a motor vehicle, it is desirable that the entire structure of an antenna device is implemented on the surface of a glass panel considering an antenna size. In this case, the antenna device is structured on one surface of a glass panel, because it is difficult to make a hole penetrating through the glass panel. An antenna formed on one surface of a glass panel is referred to as a planar antenna, one example thereof has been disclosed in Japanese Patent Publication No. 2004-214819.
Such planar antenna has been utilized for a Global Position System (GPS) antenna for receiving a signal designating a measured position from a GPS communication network for measuring the position of a motor vehicle utilizing an artificial satellite, a Dedicated Short Range Communication (DSRC) antenna utilized for a DSRC between a roadside radio equipment and a vehicle radio equipment, and an antenna for receiving a broadcast utilizing an artificial satellite or data delivered from various information service stations, for example.
In the planar antenna, the feeding point of the antenna is needed to be connected to an amplifier in a module through a coaxial feeder in order to operate an antenna device.
FIG. 1 shows a pattern of a planar antenna 8 which is composed of a hot antenna element 10 and a ground antenna element 12 surrounding the hot antenna element 10.
The hot antenna element 10 comprises an approximately rectangular opening 14 at a central portion, the outline of the hot element 10 being approximately rectangular. Two opposing corners on one diagonal line of the hot element 10 are cut away, respectively, to form perturbed portions 16a an 16b. 
The ground antenna element 12 comprises a rectangular opening 18 of a central portion, the outline thereof being rectangular. The hot antenna element 10 is located in the opening 18, and the outer periphery of the hot antenna element 10 is separated from the inner periphery of the ground antenna element 12. The planar antenna 8 is formed by a conductive material on the surface of a window glass panel of a motor vehicle.
A cavity module including an amplifier therein is mounted so as to cover the planar antenna 8. The module has a box-like shape including an opening opposed to the planar antenna 8, the inner portion thereof comprising an electronic circuitry including an amplifier. The amplifier is connected to the feeding points of the hot and ground antenna elements 10 and 12 by a coaxial feeder. These two feeding points are shown by one feeding point 19 as a representative in the figure. The module also comprises a reflective plate to concentrate a radiated energy from the planar antenna toward one direction.
The inner conductor of the coaxial feeder is connected to the hot antenna element 10 at the feeding point 19, while the outer conductor thereof is connected to the ground antenna element 12 at the feeding point 19. While respective feeding points of the hot and ground elements are provided with terminals, the attachment of the terminal to the feeding point is difficult because the size of each of the terminals is small. If a machine facility such as a robot is used for the attachment of a terminal, the manufacturing cost becomes high.
If the feeding point of the planar antenna 8 is directly connected to the amplifier in the module through a coaxial feeder, the module is not detachable from the planar antenna due to the presence of the coaxial feeder. To resolve this problem, a connector is inserted in the coaxial feeder between the feeding point of the planar antenna and the amplifier, resulting in the increasing number of components and the high cost.
In order to resolve above-described problems, a capacitive feeding method disclosed in Japanese Patent Publication No. 2004-535737 has been known in the art. According to this method, a conductive plate or electrode is located at a predetermined distance from the planar antenna, and a dielectric material is provided between the conductive plate and planar antenna to form a capacitive coupling therebetween. An electronic device (i.e., a high-frequency circuitry) is electrically connected to the conductive plate.
The capacitive coupling method described above has following problems.                1. The planar antenna is connected to a high-frequency circuitry through not a high-frequency lead but a planar electrode, so that an impedance matching to the planar antenna is difficult, which leads to a large transmission loss due to an impedance unmatching. As a result, it is required that a line connecting the planar electrode to the high-frequency is comparatively short.        2. The impedance matching is implemented by varying only the capacitance due to the simple capacitive coupling at the feeding portion. As a result, a degree of freedom for impedance matching at the feeding portion is low.        3. The larger the frequency, the smaller the size of an antenna element necessarily is. As a result, the size of the high-frequency circuitry becomes large compared to an antenna element. In such a condition, if the high-frequency circuitry integrated with the planar electrode is simply assembled at the periphery of the antenna element, then there is a fear of the distortion of an antenna radiation characteristic.        4. It is required for the simple capacitive coupling structure that the size of the planar electrode is made large or the distance between the planar electrode and the antenna element is made small in order to cause the capacitive impedance to be small. As a result, there is a fear of the occurrence of many problems in the manufacturing process.        
The object of the present invention is, therefore, to provide a feeding structure of an antenna device for a motor vehicle in which a degree of freedom for regulating the impedance matching is increased, a transmission loss at the connection to the electronic circuitry, and a radiation characteristic of the planar antenna itself is not affected.
Another object of the present invention is to provide an antenna device for a motor vehicle comprising such a feeding structure.